Streptococcus agalactiae, also known as Group B Streptococcus (GBS), is a capsulated Gram-positive bacterium that can be found in the gastro-intestinal and genito-urinary tracts of up to 40% of healthy adults. This commensal organism can be considered as a leading cause of neonatal morbidity and mortality in the developed countries (Schrag et al., 2002).
The most predictive factor of neonatal infection by GBS relates to the transmission of S. agalactiae from the mother to the neonate. This contamination generally occurs during the crossing of the genital tract, through inhalation and ingestion of contaminated amniotic liquid. It is estimated that at least 50% of newborns exposed to GBS will be colonized and that 2% will develop an infection (Schuchat, 1999). Invasive GBS infections in newborns frequently result in pneumonia and bacteraemia (approximately 80% of cases), less commonly in meningitis (10%), and death in rare instances (4%) (Schuchat, 1999). GBS-induced meningitis can lead to significant morbidity, resulting in severe neurological damage.
Approximately 80% of GBS neonatal infections occur within the first week of life and are thus designated early-onset diseases (EOD). Most of the cases of EOD are clinically apparent on the day of birth or within the first 72 hours. Late-onset diseases (LOD) usually occur in infants between 1 week and 3 months of age.
To date, nine capsular serotypes of GBS have been described: Ia, Ib, and II through VIII. Among these, serotype III GBS strains are of particular importance, as they are responsible for the majority of invasive neonatal infections and for nearly all neonatal meningitis cases in North America and Europe.
Studies also suggested that only a limited number of strains of serotype III, defined as “highly-virulent clones”, can cause a large majority of neonatal invasive diseases, and almost all meningitis cases. Recently, molecular epidemiological studies demonstrated that most GBS strains responsible for invasive neonatal infections belong to a homogeneous serotype III clone designated ST-17 (Jones et al, 2003; Luan et al, 2005).
Among the strategies set up to decrease neonatal infection by GBS, the implementation of selective intrapartum antimicrobial prophylaxis, based on either screening-strategies or risk-analyses, has lowered the incidence of EOD in the United-States and other western countries, but not that of LOD (Schrag, 2004). As progress continues to be made in EOD prevention, LOD now represents a growing proportion of all GBS neonatal infections (Schrag, 2004). Moreover, the implementation of prophylactic guidelines has increased the use of antibiotics, leading to the emergence of antimicrobial resistance in both GBS and in other perinatal pathogens.
An alternative strategy would be the early identification of highly virulent GBS in order to set up a targeted antibiotic prophylaxis.
The current recommendations to prevent Group B streptococcal disease involve screening for GBS colonization in pregnant women at 34 to 38 week gestation to identify candidates for intrapartum antibioprophylaxis. However, the culture method remains the “gold standard” technique to detect the presence of GBS in vaginal secretions. Although the culture method allows an efficient identification of GBS members, it however fails to distinguish the potentially high-virulence clones from the others.
Recent epidemiological molecular methods have proven to be powerful techniques for the characterization of phylogenetic lineages among GBS isolates pertaining to the same serotype. Among these methods, multilocus sequencing typing (MLST), multilocus enzyme electrophoresis (MLEE), pulse-field gel electrophoresis (PFGE), restriction digest pattern and restriction fragment length polymorphism (RFLP) have been especially used.
However, these techniques are fastidious and time consuming and cannot be used routinely in obstetric settings to test for the presence of highly-virulent ST-17 strains during pregnancy or at the time of delivery.
Thus, the characterization of genes specific for highly virulent GBS strains would be of major clinical importance, not only to limit the use of antibiotics, but also for the follow-up of neonates colonized by such highly virulent strains.